In an ultrasonic welding cycle, the weld phase is defined as the period during which ultrasonic energy is being applied to the parts being joined. Various conditions have traditionally been used to end the weld phase, including: (1) elapsed time from the start of the weld, (2) reaching a predetermined press position, (3) traversing a predetermined distance from the start of the weld, (4) reaching a predetermined level of ultrasound energy from the start of the weld, and (5) reaching a predetermined level of ultrasound power.
Sometimes, the weld phase is followed by a hold phase, during which the molten material cools and solidifies while the ultrasonic stack continues to be pressed against the parts being joined. Various conditions have traditionally been used to define the end of the hold phase, including: (1) elapsed time from the end of the weld, (2) reaching a predetermined press position, and (3) traversing a specified distance from the end of the weld.
These traditional methods for ending the weld and hold phases of the joining process are not adequate on some applications, particularly those where there are physical variations in the parts being welded. For example, the energy directors of the parts to be welded can vary in height, width, volume, size, and shape, due to inconsistent molding processes. The parts to be welded can also be different materials, or have other variant geometrical properties. In a first, shorter energy director, the ultrasound weld process could end the weld phase once the first workpiece has moved a set distance. However, if a second, longer energy director is welded next, and the ultrasound weld process ends based on the distance of the first workpiece, the second workpiece will not be adequately welded due to the additional time and energy needed to weld the larger energy director. The remaining conditions listed above for ending the weld phase and the hold phase all fail to account for physically variant workpieces while maintaining accurate welding.
Additionally, when an ultrasound press is operating to repeatedly weld workpieces, physical variations from a first workpiece to a second workpiece can require the ultrasound press to be recalibrated frequently. This frequent recalibration can reduce efficiency and introduce further error to the welding process, especially when the calibration is based on a first set of workpieces but the second set have different physical characteristics.
Therefore, what is needed are systems and methods for accurately welding physically variant workpieces in an ultrasound weld.